Arkoses in the centre of the hydrothermal aureole show complete illitization of feldspar and its degree of alteration decreases in the outer zone. Additionally, sandstones show illite rims around quartz grains, which are partly covered by quartz overgrowths.

Hydrothermal illites can be distinguished from detrital illites by their Mg content. Detrital illites have up to 4 wt.% MgO, hydrothermally grown illites, however, have 1 - 1.5 wt.% MgO. Illites in the centre of the aureole have relatively low total Fe contents up to 2 wt.% only, but Fe (II) increases up to 1.5 wt.% suggesting weakly reducing conditions of the mineral forming hydrothermal fluid. Si and Al are inversely correlated with high Si and low Al contents in detrital illites, but low Si and high Al contents in hydrothermal illites from the alteration centre.

K-Ar studies of the < 0.2 µm and < 2 µm fractions yield an age of 145 Ma for the hydrothermal overprint [1]. Age differences between the two fractions were found in samples with detrital illites, which implies that no complete reequilibration of detrital illites occurred and indicates alteration below the reequilibration temperature of 260±30°C of Hunziker [2].

Fluid inclusions, trapped in quartz of illite-quartz pseudomorphoses after feldspar during illitization, homogenize between 130°C and 150°C. A similar temperature is attained by vitrinite reflectance measurements, which suggest a temperature of about 150°C for the hydrothermal overprint after the calibration of Sweeney & Burnham [3].

Fluid activity in the trough is cogenetic with tectonic activities throughout Europe.